1. Field of the Invention
The invention relates to a method for ironing the wall of a deep-drawn cylindrical body in at least one thickness reduction stage. The invention also relates to apparatus for carrying out such a method, having at least one ironing ring die and a ram which forces the body through the die.
2. Description of the Prior Art
In ironing methods known in current canmaking practice, it is common for the ironing of the wall to be done in three reduction stages. This is illustrated by FIG. 1 of the drawings accompanying the present application. See also EO-A-5084, GB-A-2112685, GB-A-2155378 and GB-A-2181082. For processes apparently involving two reduction stages see GB-A-1345227, US-A-3423985, US-A-4038859 and US-A-4173882. Immediately prior to the reduction stages, there may be a drawing die, in which the ram or punch reshapes a flat metal blank into a body with a cylindrical wall.
The cylindrical body formed by ironing the wall is filled with a product e.g. a beverage, generally after applying a layer of varnish to the inside, after which a lid is joined to the flanged edge at the open end of the cylinder body.
Constant endeavours are made to reduce the cost of packaging the product, i.e. the costs of the cylindrical body and the lid.
One of the possibilities is to reduce the quantity of material from which the cylindrical body is made. The usual dimensions for a beverage can (see EP-A-122651) are a bottom thickness of about 0.3 mm, a wall thickness of about 0.10 mm and a flange at the open end of the cylindrical body with a thickness of about 0.16 mm.
Reducing the quantity of material in such a can while maintaining the capacity is only possible by reducing the thickness dimensions of the can. Endeavours are particularly directed towards reducing the wall thickness. In known methods this is however not easily possible; this is to be attributed especially to the size of the reduction stages which are required to produce a cylindrical body with a smaller wall thickness. Large reduction stages in fact rapidly lead to the formation of cracks during ironing of the wall. It has also appeared that the cylindrical body, depending on the combination of reduction stages, acquires an undesirable, for example, dull, external appearance.
It is known to have two thickness reduction stages occurring simultaneously in two successive ironing dies. This is apparantly the case in EP-A-5084 and US-A-4038859 mentioned above. Where both thickness reductions are substantial, this requires very large ram forces, which tend to damage the can. It is also known, from GB-A-1345227 to employ an ironing die structure in which two die rings are closely adjacent and the first die ring effects only a pre-sizing or slight thickness reduction prior to the main thickness reduction in the second die ring. This pre-sizing is said to amount to "skimming off" of any areas of excessive thickness of the cylindrical wall.